incubator help.....please

okay im using an old maha steaks cooler a 40w light bulb and the same water heater thermo that i have seen everyone else using. my problem is the big fluctuation in temps when the bulb is on and off. the temps will be 101.1 midrange before thermostat kicks in 92.8 thermostat shuts off at 105.3 i cant get a stable temp. whats wrong or is this normal?

You need to turn your fan to where it blows the heat off the light strait on to the thermostat. Turning the flat side of the thermostat in the direction of the light helps too. You wont the thermostat to get hot an kick of very fast after the light comes on. If you don't have a fan get one. Also put bricks or jars of water in any spare spaces you have.

Are you checking your temps with a probe in a water wiggler? That pretty much mimics the temp inside an egg. I placed my light bulb in the center of one end of a Coleman cooler. To the left of the light bulb was the water heater thermostat and to the right of the bulb was the fan diagonally across the corner of the cooler so that it blew air across the 40w bulb and towards the thermostat. A heatsink helps regulate the temps, too (a canning jar of warm water or something similar).

There are some that manage to keep temps in reasonable ranges wtih a water heater thermostat. There are many that find them just to variable in terms of holding temps. If you think about the range of temp that comes out of your water heater the highs and lows are quite variable. I know there is a lot of variation with cold water flowing in etc. but still, I think you could save yourself a bunch of brain damage with a wafer thermostat about 20 bucks.